Active sodium (Na+) transport by distal lung epithelium plays a critical role in maintaining normal alveolar fluid balance and in preventing pulmonary edema. The effect of inhaled nitric oxide (NO) on lung epithelial Na+ transport is largely unknown. We have used the patch-clamp technique to study the effect of NO on a Na+ permeant cation channel in rat type-II pneumocytes (ATII cells). Single-channel recordings from the apical surface of ATII cells in primary culture showed a predominant cation channel with unitary conductance of 17.6 pS and Na+/K+ selectivity of 0.9. Nitric oxide donor GSNO (100μM) inhibited the basal cation channel activity by 56% (nPo control vs. GSNO, x±SEM, 0.29±.07 vs. 0.13±.04, n=6, p <0.05). The effect was rapid with onset of less than 2 min and sustained through up to 30 min of recording. GSNO inhibition was reversed by washout, and the unitary conductance remained unchanged. The inhibitory effect of NO was confirmed by using a second donor of NO, SNAP (100μM) (0.45±.09 vs. 0.26±.06, n=6, p <0.05). The GSNO effect was blocked by methylene blue (100μM) suggesting a role for cGMP. The soluble analog of cGMP, 8Br-cGMP (100μM) inhibited the cation channel similar to that seen with GSNO (nPo control vs. Br-cGMP, 0.35±.08 vs. 0.17±.06, -52%, n=6, p <0.05). To determine if the activation of cGMP-dependent protein kinase was necessary for NO to inhibit Na+ channels, cells were treated with KT 5823 (a blocker of protein kinase G) prior to treatment with GSNO. KT 5823 abolished the inhibitory effect of GSNO. The NO inhibition of channels was not due to changes in cell viability. We conclude that NO suppresses the activity of a Na+ permeant cation channel on the apical surface of ATII cells. This action appears to be mediated by cGMP-induced activation of a protein kinase. The modulation of Na+ channels by NO may have important clinical implications since it can adversely effect alveolar fluid balance resulting in the development of pulmonary edema.